This invention relates to an apparatus and method to calibrate one or more servo sensors in a xe2x80x9cnoisyxe2x80x9d environment. In certain embodiments, this invention relates to servo track following a moving magnetic tape having one or more servo edges of dissimilar recorded servo signals, and, more particularly, to calibrating one or more servo sensors with respect to one or more indexed servo positions offset laterally from those one or more servo edges.
Automated media storage libraries are known for providing cost effective access to large quantities of stored media. Tape cartridges containing a moveable magnetic tape are often used in automated data storage libraries. Tape media, such a magnetic tape, is a common medium for the storage of data to be utilized by a computer. Magnetic tape has found widespread use as a data storage medium because it provides a relatively inexpensive solution for storing large amounts of data.
Magnetic tape data storage typically provides one or more prerecorded servo tracks to allow precise positioning of a tape head with respect to those prerecorded servo tracks. Servo sensors disposed on the tape head are used to track the recorded servo tracks. The tape head comprises one or more read/write elements precisely positioned with respect to those servo sensors. One example of a magnetic tape system is the IBM 3590, which employs magnetic tape having prerecorded servo patterns that include three parallel sets of servo edges, each servo edge being an interface between two dissimilar recorded servo signals, each set of servo edges comprising one servo edge on each of opposite lateral sides of a middle recorded servo signal.
In certain embodiments, the tape head includes a plurality of servo sensors for each servo edge, with the result that the tape head may be stepped between those servo sensors, each positioning the read/write elements at different interleaved groups of data tracks. Typically, for a given servo pattern of a set of two servo edges, the outer servo signals are recorded first, and the center servo signal is recorded last, to provide the servo edges. The nominal separation distance between the servo edges of each set of servo edges is a certain distance, but there is variation in the magnetic separation between the servo edges, for example, due to the variation of the width of the physical write element which prerecords the servo pattern, due to variation in the magnetic characteristics of the physical write element, etc. The variation may occur between servo tracks in a single magnetic tape, and may occur between prerecording devices and therefore between magnetic tapes.
To reduce the apparent difference of the edge separation distance of the prerecorded servo tracks from nominal, the prerecording of the servo tracks is conducted at different amplitudes so as to attempt to compensate for the physical difference and provide a magnetic pattern that is closer to nominal. Thus, the difference in physical distance and the amplitude compensation may tend to offset each other with respect to the apparent distance between the servo tracks. These actions may provide an adequate signal for track following at the servo edges.
However, to increase track density, a servo sensor may be indexed to positions laterally offset from the linear servo edges to provide further interleaved groups of data tracks. The indexed positions are determined by measuring the ratio between the amplitudes of the two dissimilar recorded servo signals. Thus, when the amplitudes of the recorded servo signals are varied to compensate for physical distance variations, track following the prerecorded servo edges at the offset indexed positions becomes less precise. As the result, the data tracks may vary from the desired positions, i.e. be xe2x80x9csqueezedxe2x80x9d together, such that writing on one track with a write element that is subject to track misregistration (TMR) may cause a data error on the immediately adjacent data track.
The tape path of the above IBM 3590 is a guided tape path. In such a guided tape path embodiment, the magnetic tape can be moved in a first direction and an opposing second direction along a first axis, i.e. along the longitudinal axis of the tape. Movement of that tape along a second axis orthogonal to the first axis, i.e. the lateral axis of the tape, is minimized. Limiting the lateral movement of the magnetic tape results in generating minimal guiding noise, and therefore, the step from a first ratio of servo signals to a second ratio is readily discernible.
Another approach, however, is required for open channel guiding in which the magnetic tape can move laterally a distance which is substantially greater than the separation between index positions, thereby introducing substantial noise into the guiding process. The guiding signal to noise ratio thus becomes negative, with the guiding noise being far larger than the step from one ratio to another, making it difficult to gather data points with a monotonic slope to conduct a calibration of the servo ratios.
Applicants"" invention includes an apparatus and method to calibrate a servo sensor disposed on a magnetic tape head disposed adjacent a magnetic tape moving along a tape path. The magnetic tape includes at least one servo edge comprising an interface between a first recorded signal and a second recorded signal. The servo sensor is capable of detecting that first recorded signal and that second recorded signal. Applicants"" apparatus includes an independent position sensor which provides an IPS signal comprising the lateral position of the tape head with respect to the tape path. Applicants"" method includes a coarse initial calibration of the servo sensor which is performed before performing the complete calibration.
Applicants"" method first positions the tape head while slewing through the servo pattern written to the magnetic tape. By xe2x80x9cslewing through the servo patternxe2x80x9d Applicants mean moving the tape head in a substantially linear manner through most or all of the range of motion where meaningful servo signal ratio data exists for a given group servo reader elements. Starting from a position where the servo read elements are entirely off their corresponding servo edges of interest in one direction, i.e. from the outside of the servo pattern, the tape head is smoothly moved to a position where the servo read elements are entirely off their corresponding servo edges of interest in the opposite direction, i.e. toward the center of the servo pattern and the opposite servo edge. In certain embodiments, Applicants"" method slews the tape head from a high servo signal ratio to a low servo signal ratio.
Applicants"" method provides a servo signal during that tape head positioning, where that servo signal comprises the ratio of the detected first recorded signal and the detected second recorded signal. Using that detected servo signal, Applicants"" method generates initial servo signal information. Concurrently, Applicants"" method provides an IPS signal during positioning of the magnetic tape head. Using that IPS signal, Applicants"" method generates initial IPS signal information. Using the initial servo signal information and the initial IPS signal information, Applicants"" method calculates a first transfer function prior to moving the magnetic tape.
Applicants"" method includes establishing a maximum allowable residual error REMAX for the calibration process. After calculating the first transfer function, Applicants"" method determines the first residual error for that first transfer function. Applicants"" method then compares the first residual error with the maximum allowable residual error. If the first residual error is less than
REMAX, then Applicants"" method saves the first transfer function for subsequent use. Alternatively, if the first residual error is equal to or greater than REMAX, then Applicants"" method uses the first transfer function to calibrate the servo sensor using either known or novel calibration procedures which include moving the magnetic tape.
Applicants"" invention further includes an article of manufacture comprising a computer useable medium having computer readable program code disposed therein to calibrate a servo sensor disposed on a tape head disposed adjacent a magnetic tape prior to moving that magnetic tape. Applicants"" invention further includes a computer program product usable with a programmable computer processor having computer readable program code embodied therein to calibrate a servo sensor disposed on a tape head disposed adjacent a magnetic tape prior to moving that magnetic tape.